The present invention relates to a novel superficially mixed metal oxide electrode and the process by which it is made. The electrode is useful in anodic electrochemical reactions, particularly the anodic electrochemical oxidation of chloride to chlorine in an aqueous solution.
A variety of materials have been utilized in chlorine anodes starting with graphite. But because of several problems inherent to graphite, most notably its high overpotential and relatively low corrosion resistance, catalytically active noble metals and noble metal oxides were used. While successfully overcoming the disadvantages of graphite, such electrodes are extremely expensive.
Various attempts have been made to reduce or replace the noble metals in chlorine anodes. The use of a conductive tin oxide layer consisting only of mixtures of tin oxide and antimony is disclosed in U.S. Pat. No. 3,627,669, but a conductive tin oxide layer exhibits extremely high overpotential for chloride oxidation. One method recently developed to utilize decreased amounts of noble metal is that illustrated in U.S. Pat. No. 3,711,385. This is, essentially, coating the valve metal substrate with a thin film or coating of a noble metal or noble oxide which is preferably a mixture of platinum with another noble metal or noble oxide. The difficulties experienced with these electrodes are discussed in U.S. Pat. No. 3,882,002 to Cook and it is because of those difficulties that Cook developed his electrode as described in the patents listed below.
Typically, as shown in a review of U.S. Pat. Nos. to Cook, 3,882,002, 3,951,766, 3,986,942, 3,940,323, 3,943,024 and 3,956,083, electrodes to be used in electrolytic processes are comprised of a valve metal substrate, normally titanium, coated with conductive tin oxide and which have an outer coating of a noble metal or noble metal oxide. Such an electrode is normally prepared by depositing an adhering layer of tin oxide on the titanium base. Preferably, a minor amount of dopant, for example, an antimony compound is also co-deposited along with the tin oxide forming a conductive tin oxide. The antimony stabilizes and lowers the electrical resistivity of tin oxide compositions. The tin oxide along with the dopant may be adherently formed on the titanium base in a number of ways. For example, the titanium base can be sprayed, painted, brushed or otherwise coated with an aqueous solution of a thermally decomposable salt containing, for example, a salt of tin and a salt of a suitable dopant such as antimony. The coating is then dried by heating, for example at about 100.degree. to 200.degree. C., for a few minutes to evaporate the solvent and then at higher temperatures, e.g., 250.degree. to 800.degree. C. in an oxidizing environment to convert the tin and antimony to their respective oxides.
Optionally, small amounts of a chlorine discharge catalyst such as manganese difluoride may also be added to this tin oxide layer to lower the overpotential required for chlorine gas liberation.
The electrodes prepared in accordance with the above-listed Cook patents have an outer layer consisting of either a noble metal or noble metal oxide. This outer layer is deposited over the conductive tin oxide layer by such known methods as electroplating, chemical deposition from a platinum coating solution, or spraying. A noble metal oxide outer layer can be made by depositing the noble metal in the metal state followed by oxidation, for example, by galvanic or chemical means or by heating at elevated temperatures of from about 300.degree. to 600.degree. C. or higher in an oxidizing atmosphere. A preferred method for the formation of the noble metal oxide layer involves coating the conductive tin oxide layer with a solution of a noble metal compound, evaporating the solvent, and then oxidizing the noble metal at elevated temperatures between about 300.degree. and about 800.degree. C. in an oxidizing atmosphere.
electrodes prepared by this known method, while demonstrating good overpotential properties in, for example, the anodic electrochemical oxidation of chloride in an aqueous medium to chlorine, still use a substantial amount of noble metal in making the necessary outermost layer.
It is, therefore, an object of this present invention to make an electrode with good overpotential properties, but which contains only a minimal amount of noble metal.
It has been discovered that a superficially mixed metal oxide electrode can be prepared in accordance with the present invention which has significantly less noble metal present while, surprisingly, retaining favorable overpotential properties necessary for chlorine oxidation.
For purposes of the present invention, a valve metal and a film-forming metal are essentially the same and mean one of the metals, titanium, zirconium, niobium, tantalum, and tungsten or an alloy consisting mainly of these elements and having anodic polarization properties similar to the commercially pure elements as known in the art. For electrodes used in the anodic electrochemical oxidation of chloride to chlorine in an aqueous solution, the preferred film-forming metals are titanium and alloys which are based on titanium and have anodic polarization properties comparable to those of titanium.